diff --git a/apps/desktop/src-tauri/src/main.rs b/apps/desktop/src-tauri/src/main.rs
index 57f0039..9ac1ec1 100644
--- a/apps/desktop/src-tauri/src/main.rs
+++ b/apps/desktop/src-tauri/src/main.rs
@@ -342,6 +342,14 @@ fn validated_result(index: &SearchIndex, path: &str) -> Result<PathBuf, String>
     Ok(canonical)
 }
 
+fn preview_pixel_limit(format: Option<image::ImageFormat>) -> u64 {
+    if format == Some(image::ImageFormat::Jpeg) {
+        64 * 1024 * 1024
+    } else {
+        16 * 1024 * 1024
+    }
+}
+
 #[derive(Serialize)]
 struct MediaPreview {
     path: String,
@@ -373,21 +381,15 @@ async fn prepare_media_preview(
             if bytes > 64 * 1024 * 1024 {
                 return Err("Image exceeds the 64 MiB preview limit.".into());
             }
-            let (width, height) = image::ImageReader::open(&canonical)
+            let reader = image::ImageReader::open(&canonical)
                 .map_err(|error| error.to_string())?
                 .with_guessed_format()
-                .map_err(|error| error.to_string())?
+                .map_err(|error| error.to_string())?;
+            // Resource limits follow decoded format, not a renameable suffix.
+            let max_pixels = preview_pixel_limit(reader.format());
+            let (width, height) = reader
                 .into_dimensions()
                 .map_err(|error| format!("Image preview is unavailable: {error}"))?;
-            let jpeg = canonical
-                .extension()
-                .and_then(|value| value.to_str())
-                .is_some_and(|value| matches!(value.to_ascii_lowercase().as_str(), "jpg" | "jpeg"));
-            let max_pixels = if jpeg {
-                64 * 1024 * 1024
-            } else {
-                16 * 1024 * 1024
-            };
             if width == 0 || height == 0 || u64::from(width) * u64::from(height) > max_pixels {
                 return Err("Image exceeds the supported preview dimensions.".into());
             }
@@ -809,4 +811,26 @@ mod tests {
         assert!(validated_result(&index, &indexed.to_string_lossy()).is_err());
         assert!(validated_result(&index, &root.path().to_string_lossy()).is_err());
     }
+
+    #[test]
+    fn renamed_png_cannot_receive_the_larger_jpeg_preview_budget() {
+        let root = tempfile::tempdir().unwrap();
+        let path = root.path().join("misnamed.jpg");
+        let image = image::GrayImage::new(4097, 4096);
+        image
+            .save_with_format(&path, image::ImageFormat::Png)
+            .unwrap();
+        let reader = image::ImageReader::open(&path)
+            .unwrap()
+            .with_guessed_format()
+            .unwrap();
+        assert_eq!(reader.format(), Some(image::ImageFormat::Png));
+        let limit = preview_pixel_limit(reader.format());
+        let (width, height) = reader.into_dimensions().unwrap();
+        assert!(u64::from(width) * u64::from(height) > limit);
+        assert!(
+            u64::from(width) * u64::from(height)
+                < preview_pixel_limit(Some(image::ImageFormat::Jpeg))
+        );
+    }
 }
diff --git a/docs/ROADMAP.md b/docs/ROADMAP.md
index 4dd288d..f8bc95c 100644
--- a/docs/ROADMAP.md
+++ b/docs/ROADMAP.md
@@ -53,6 +53,11 @@ release. Keep their evidence and unchecked production gates intact.
   see [incremental semantic design](INCREMENTAL-SEMANTIC.md).
 
 ## 3. Desktop
+- [x] Image preview pixel ceilings use actual decoded format; a real PNG renamed
+  `.jpg` cannot receive the larger JPEG allowance (Windows CPU regression).
+- [ ] Replace lifetime-growing native media asset grants with bounded preview
+  access, including range limits, stale-source handling and selection/reload tests.
+  Do not use permanent forbidden patterns as revocation.
 - [x] Official single-instance transport with isolated explicit profiles;
   Windows CPU duplicate launch exits without a second query worker.
   See [launch evidence](SINGLE-INSTANCE.md); visual focus and other OS checks remain open.